THE MOSAIC STAGE OF DIFFERENTIATION 195 



referred to as chemo-differentiation (see p. 46). ^ It now becomes 

 necessary to consider this phase of development in greater detail. 



Experiments and operations on early stages (early tail-bud) of 

 embryos of Urodeles (Amblystoma) have now shown that, beyond 

 mechanical wound healing, no regeneration or regulation occurs."^ 

 If, for instance, the embryo is cut into two by a transverse section, 

 the two portions continue their prospective development, the front 

 portion forming a head and neck region, the posterior portion a 

 trunk and tail. The number of external gills on the one or the other 

 portion depends upon the precise position of the cut (fig. 92). 



Similarly, it has been found that the anterior third of a 24-hour 

 blastoderm of a chick embryo grafted on to the chorio-allantois of 

 another egg gives rise to just those organs which it would have 

 produced in normal development^ (fig. 93). Two half-embryos 

 of frogs grafted together will develop into a single frog, even if the 

 halves belong to diiTerent species. Each half retains its specific 

 characteristics (see p. 406, and fig. 196). 



In Amphibians, such fragments of course cannot develop far 

 beyond hatching. If, however, in the early tail-bud stage, the tip 

 of the tail is cut oflF, the organism develops into a healthy larva, but 

 with a permanently shortened tail.^ Removal of rudiments of eyes, 

 gills, limbs, heads, or snouts at this stage results in permanent 

 absence of these structures in the later embryo and larva. Similarly, 

 experiments on the fish Fundulus have shown that removal of 

 portions of the embryonic shield results in permanent absence of 

 the structures whose rudiments have thus been affected^ (fig. 94). 



More recent and detailed work has shown that in Triton at the 

 stage when the tail-bud is hemispherical, complete or almost com- 

 plete removal of the mesodermal contents of the bud results in 

 completely tailless larvae, whereas in only slightly later stages 

 complete regeneration can and does occur.^ Further, by appro- 

 priate operations, more localised defects can be obtained, e.g. 

 absence of ventral fin membrane, of mesodermal somites, of noto- 

 chord, or of nerve tube. When regeneration experiments are 

 carried out on such partially defective tails in the larval stage, it is 



^ Huxley, 1924; Goldschmidt, 1927; Bertalanffy, 1928. 

 2 Schaxel, 1922 b. ^ Murray and Huxley, 1925. 



* Eycleshymer, 1914; Nicholas, 1927; Hoadley, 1928. ^ Vogt, 1931. 



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